1. Field of the Invention
This invention relates to a system for processing a substrate with plasma and in particular to a plasma processing system such as a plasma enhanced chemical vapor deposition (PECVD) system or a plasma etching system.
2. Description of the Related Art
To manufacture a semiconductor integrated circuit, a liquid crystal display or the like, plasma is used to apply predetermined processing to the surface of a substrate. FIG. 11 is an illustration showing a schematic configuration of a conventional plasma processing system used for such processing.
The plasma processing system shown in FIG. 11 consists mainly of a vacuum vessel 1 provided with an exhaust channel 11, a gas introduction mechanism 2 for introducing a predetermined gas into the vacuum vessel 1, a power supply mechanism 3 for energizing the introduced gas for forming plasma, and a substrate holder 4 for placing a substrate in a position where the substrate is to be processed with the formed plasma.
The system in FIG. 11 carries a substrate 40 into the vacuum vessel 1 through a gate valve (not shown) and places the substrate 40 on the substrate holder 4. After evacuating air in the vacuum vessel 1 through the exhaust channel 11, the system introduces a predetermined gas by the gas introduction mechanism 2. Next, the system applies energy of high-frequency electromagnetic wave power and the like, to the gas in the vacuum vessel 1 by the power supply mechanism 3 for forming plasma. Then, the system applies predetermined processing to the surface of the substrate 40 with the formed plasma. For example, if mono-silane gas and oxygen gas are introduced by the gas introduction mechanism 2, the plasma causes a decomposition reaction and the like, and PECVD processing can be performed for depositing a silicon oxide thin film on the surface of the substrate 40.
When the processing for the substrate 40 is repeated a considerable number of times in the conventional plasma processing system, a phenomenon appears in which a thin film is also deposited on the surface of the portion exposed to the plasma in the surface of the substrate holder 4 (which will be hereinafter referred to as exposure face in the specification), and the inner face of the vacuum vessel 1. For a film depositing system such as a PECVD system, the deposited film is often the same as the thin film deposited on the substrate. For a plasma etching system, etched material may adhere and grow into a thin film.
If such a thin film reaches a considerable film thickness, it peels off due to the internal stress of the thin film. The peeled thin film causes fine powder drifting in the vacuum vessel to occur. If the fine powder adheres to the substrate 40, it causes a surface blemish to occur, degrading the commodity value of the thin film deposited on the surface of the substrate or causing a serious circuit failure to occur.
To suppress such deposited film peeling, a plasma cleaning technique is generally used for etching and removing the deposited film before peeling. This technique is executed during intervals between substrate processing steps. In the technique, for example, mixed gas of perfluorocarbon 14 gas (CF.sub.4):oxygen gas=80:20 or so is introduced into the vacuum vessel 1 by the gas introduction mechanism 2, the power supply mechanism 3 is operated for forming plasma of the mixed gas, and the deposited film is etched by radical CFx (x=1, 2, or 3), CFx ions (x=1, 2, or 3), radical fluorine, or fluorine ions generated in the plasma. That is, fluorine radicals or ions react with the deposited thin film to yield volatile substance, which is then discharged through the exhaust channel 11, thereby removing the thin film.
However, even if such plasma cleaning as described above is executed, it is difficult for the configuration of the conventional plasma processing system to etch and remove the thin film deposited on the exposure face of the substrate holder; even if the plasma cleaning is executed for a predetermined time, a thin film often remains on that portion. If an attempt is made to completely remove the thin film on the exposure face, the plasma cleaning must be executed for a long time. The fact that it takes a long time in such operation performed during intervals between substrate processing steps will cause the system availability to lower.